The present invention relates to disc screen apparatuses for sorting, by size, particulate matter such as wood chips, and the like. It more specifically relates to discs for such apparatuses and methods for forming such discs.
Disc screens have been used for many years to sort a variety of objects by size, such as wood chips, coal, coke, grain, beets, leaves, sticks and potato chips. For example, uniform high yield wood pulp requires correctly sized and composed wood chips. Examples of disc screens are those shown in the following U.S. patents. These patents and all other patents and publications mentioned herein are hereby incorporated by reference in their entireties.
______________________________________ U.S. Pat. No. Patentee ______________________________________ 4,037,723 Wahl et al. 4,239,119 Kroell 4,301,930 Smith 4,376,042 Brown 4,377,474 Lindberg 4,452,694 Christensen et al. 4,538,734 Gill 4,579,652 Bielagus 4,653,648 Bielagus 4,658,964 Williams 4,658,965 Smith 4,703,860 Gobel et al. 4,741,444 Bielagus 4,755,286 Bielagus 4,795,036 Williams ______________________________________
Generally speaking, these disc screens include a frame and a plurality of rotating parallel shafts mounted within the frame. Each of the shafts has a plurality of spaced apart discs mounted thereon. The discs on adjacent shafts intermesh and rotate side-by-side with a fixed critical distance between the intermeshed discs. These disc screens typically have an entrance end perpendicular to the longitudinal axes of the shafts. Opposite the entrance end is an exit end which is adjacent to a discharge port. Each shaft rotates in a downstream direction to transport matter along the discs from the entrance end to the exit end.
In operation, the particulate matter to be sorted is dropped from above the disc screen along the entrance end. The downstream shaft rotation carries the larger pieces of particulate matter across the upper surface of the screen to the discharge port. The smaller size particulate matter falls due to gravity through the critical fixed distance spaces between the intermeshing discs for collection below the disc screen. Generally, the shafts of the disc screens are coplanar and rotate in a horizontal plane.
Some devices have utilized the disc screen in an inclined position. For example, if the entrance end is at a higher level, gravity assists in transporting the larger particles over the upper surface of the disc screen. Other disc screen arrangements have linked inclined and horizontal disc screen sections, with a continuous path of travel along the upper surface of the linked sections.
The critical spacing between the intermeshing discs depends upon the disc spacing along adjacent shafts. Various methods have been used to maintain the required disc spacing on a given shaft. Many devices utilize spacers, such as washers, between adjacent plate-like disc. Close axial tolerances must be maintained on both spacers and discs to minimize the cumulative error over the length of a shaft. Close tolerance requirements, however, increase the cost of such assemblies.
Other devices use discs having hubs projecting outwardly from one or both sides of the disc which butt against the adjacent hub or disc. Some hubbed discs are die cast and susceptible to fracture from porosity and other material impurities. Die cast discs are generally thicker, heavier to handle, and expensive due to the increased material required, however. Many of these earlier devices have used bearings having cast bearing housings to mount the rotating shaft to the frame. These cast bearing housings usually have oversized mounting bolt holes to facilitate shaft alignment. Vibrations encountered during operation can loosen the mounting bolts, allowing the bearing housing to shift. Thus the critical spacing is not maintained.
For shaft assemblies having a plurality of spaced apart discs mounted upon a cylindrical shaft, there is an undesirable tendency for the discs to rotate relative to the shaft and/or relative to each other. This undesirable rotation impedes the flow of the particulate matter across the screen. A variety of notch and key methods have been used to prevent this rotation. Examples thereof are shown in the previously-listed '723 patent to Wahl, the '734 patent to Gill and the '119 patent to Kroell. Another method has been to weld the discs to the shaft to prevent the rotation and maintain axial alignment. The welding of the discs is a time consuming process, however, due to the close tolerances often involved and may also heat warp the discs. A prior art disc and disc screen assembly which remedies many of the problems has been commercially available from Mill Services and Manufacturing, Inc. of Hattiesburg, Miss. under the trademark "SoloDisc," which can be used in a flat screen or a V-screen replacement shaft assembly. This prior art disc screen assembly allows the discs to be readily fitted upon a shaft during initial assembly, retrofitting and replacement. The shaft assembly has a minimal number of parts and has minimal disc wobble resulting. This prior art system is illustrated in FIGS. 1-5 generally at 100 and is described below.
Referring to FIGS. 1 and 2 it is seen that a shaft 102 is driven by a belt drive (a "Gates Poly Chain GT" drive--see e.g., U.S. Pat. No. 4,605,389) extending over a sheave 104. The belt drive thereby directly drives the entire shaft (a "live" shaft) through a bearing 106 and which in turn drives a pipe roll 108. Thus the pipe roll 108 is secured to and rotatable with the shaft. A plurality of individual stepped discs 110 are slipped into place on the roll and held therein by the locking slots, by the stepped relation of the discs, and by the compression lock nut 112 securable thereto. Also illustrated in the FIG. 2 are the male fixed end cap 114, the female compression ring 116, the lock washer 117 and the shaft seal 118 of shaft assembly 100.
The prior art disc 110 shown in isolation FIGS. 3-5, comprises a disc plate 122 having teeth 124 about its outer perimeter and a double stepped spacing and nesting sleeve shown generally at 126 integrally formed with the plate. The first step 128 is sized diametrically to slidably receive the tubular shaft (108). The second step 130 interconnects the first step 128 with the plate 122 and is diametrically sized to slidably receive the first step of a preceding disc. Thus, the adjacent precedingly and subsequently assembled discs are nested together by their overlapping steps. This sleeve 126 spaces the discs 110 at the desired distance.
A stop 132 and a stop engaging surface 134 are provided on the two-stepped sleeve 126. The stop 132 is formed as a shoulder defined by the outer radius of the bend in the sleeve 126 connecting the second step 130 with the disc plate 122. The stop engaging surface is shown by the shoulder stop 132 located at the outer periphery of the diametrical transition between the first and second steps 128, 130. When assembled, the shoulder of one disc engages the shoulder stop of the adjacent disc to maintain a desired spacing between adjacent discs. Separate spacers are thus not required to maintain disc spacing. The desired spacing is determined from the disc plate thickness and the maximum size of acceptable particles. In other words, the critical space equals one-half the difference between the desired spacing and the disc thickness. Thus, the axial length of the first step on a preceding disc should be long enough to extend under the second step but not so long as to interfere with the first step of the next disc.
After the discs have been assembled on the shaft, the lock nut 112 is tightened, forcing the compression collar inward and the shoulders of the discs into engagement with the shoulder stops of the preceding discs. The lock washer 117 prevents further rotation of the lock nut 112 and maintains the axial alignment of the discs. Thus no welding, which is not only time consuming but may also heat warp the disc, is needed. Each of the discs is provided with an inwardly projecting key or dimple 140 on the first step 128 of the disc which then slides onto a longitudinal groove on the outer surface of the tubular shaft thereby preventing relative rotation of the discs.
These discs were manufactured from ductile steel using a three-step draw die. At the first draw die step the center opening was punched through the disc, a slight draw of around 71/2 millimeters for the double stepped sleeve was formed and the outer diameter of the disc was punched. The second die punched the draw to form the double stepped sleeve. A third punch formed the teeth on the outer diameter of the disc. These teeth were chrome plated in a subsequent operation. In a fourth forming step a slide punch placed the anti-rotation dimple or key in the first step of the sleeve. The disc assemblies were assembled with the teeth on adjacent discs staggered to assist in pulling apart the mat of particulate matter conveyed thereon. The dimple was thus located positively or fixed relative to the teeth. (Examples of prior art die stamping procedures for other articles are disclosed in U.S. Pat. Nos. 3,707,133 and 3,834,212.)
Thus with the discs slid into place on the shaft and held together by the end clamping means, the stop of one disc is adjacent the shoulder stop of the adjacent disc. Since both of these surfaces are curved rounded surfaces, as best shown in FIG. 5, the contact between the shoulder and the stop, when viewed in cross-section, is essentially only a point contact, or when viewed in three dimensions is a circle line contact, the line having a maximum width of generally only one thirty-second of an inch. This provides for only a ball joint type of coacting relationship, allowing one surface to roll against the other, that is, allowing the discs to wobble.
Although as a practical matter this prior art screen functioned effectively, commercially they were not as successful as desired due to this wobble. The customer requires uniform spacing with extremely tight tolerances, and IFOs having an accuracy of twenty thousandths of an inch are preferred. No method, even the "SoloDisc", was known for consistently providing these accurate IFOs in a system without any undesirable wobbling of the disc.
In fact since such a system was thought not possible, the trend in sorting machines has been away from disc screens and to spiral and diamond roll-type screens. Examples of such are the "DynaGage Bar Screen" available from Rader Companies, which is a division of Beloit Corporation and has a headquarters in Portland, Oreg. It includes z gauge bars. The slots between the bars establish the maximum particle thickness that will pass through the screen. When activated the eccentricity of the shafts causes each deck to oscillate independently. Another recent design also available from Rader Companies is the "Raderwave Fines Screen", which has a series of parallel shafts located beneath a flexible perforated screen deck. A wave-like motion is created on the material on the screen when the shafts rotate. The pins and chips are thereby apparently suspended, the fines (undersized chips) migrate through the perforations and acceptable fiber travels across the screens.
Another example is the "ChipManager PST" available from Evergreen Engineering, Inc. of Eugene, Oreg. and disclosed in U.S. Pat. No. 4,376,042. A further system also available from Evergreen Engineering is their "ChipManager VSF". It uses a small horizontal disc screen head of an existing system to thereby split the infeed mass and more thoroughly remove fines and overthicks.